1. Field of the Invention
The present invention relates to devices for controlling, generating and guiding waves. The waves can be electromagnetic waves, acoustic waves, and the like. Such a device comprises at least two adjacent media with a cylindrical interface or a disk-like thin-film waveguide with at least one ridge and one groove on one of the thin films. One of the media can be air or vacuum. The device may take the form of a multi-cylindrical-layer wrapped rod, or a disk-like slab-waveguide (thin film) structure with circular ridge-groove grating in one of the slabs. The cylindrical-layer and grating-step spacings are critical in designing the devices.
2. Description of the Prior Art
About a hundred years ago, it was observed that a periodic crystal lattice structure can reflect X rays. W. L. Bragg was the first to propose a reasonable explanation for this phenomenon. Since then, plane (one-dimensional) periodic structures, constructed by alternating plane dielectric layers (sheets), have been used as high-reflection coatings for optical components and devices and as mirrors for optical waves in narrow-bandwidth filters, vertical-cavity surface-emitting semiconductor lasers (VCSELs) and many other applications. Such structures are often called distributed-Bragg reflectors (DBRs) or multi-stack dielectric mirrors and are among the most important building blocks for wave generation and control devices.
Altered forms of such structures, taking the form of corrugated gratings in waveguides, are applied to guided optical waves. They are used in semiconductor DBR lasers, quarter-wavelength-shifted distributed-feedback (DFB) lasers, waveguide filters and other guided-wave devices. The ridges and grooves of the gratings induce periodic changes in the effective index of the waveguides, and thus function as distributed reflectors to the guided waves.
The layer spacings of DBRs are usually odd-integer multiple of a quarter of the designated wavelength. Multi-layer structures, with spacings of odd and even multiples of a quarter wavelength, have found other uses, for example anti-reflection coatings for optical components and devices. The guided wave versions of such structures, with gratings to create the alternating changes in effective index, have been found to function as directional couplers that couple optical waves out of waveguides.
Another use of the periodic grating structures, mostly used in DFB lasers, takes advantage of the guided wave modes that experience equal forward and backward reflectivities from the gratings. DFB lasers (with or without quarter-wavelength shifting) and DBR lasers are the most important semiconductor lasers that can achieve narrow linewidths, or high spectral purity in other words.
DBR and DFB lasers are used widely in telecommunication and cable TV distribution systems. VCSELs are important for optical interconnect applications. High-reflection and anti-reflection coatings are usually used in optical components and devices which contain media interfaces.
There have been many efforts to construct cylindrical (two-dimensional) versions of DBR, anti-reflective, directional coupler and DFB structures for cylindrical wave devices. Most of these efforts are direct extensions of the one-directional versions. The following typical references represent the large body of the prior art.
[1.] U.S. Pat. No. 4,743,083 to R. M. Shimpe, "Cylindrical diffraction grating couplers and distributed feedback resonators for guided wave devices," May 10, 1988.
[2.] T. Erdogan and D. G. Hall, "Circularly symmetric operation of a concentric-circle-grating, surface-emitting, AlGaAs/GaAs quantum-well semiconductor laser," Appl. Phys. Lett., vol. 60, pp. 1921-1923, 1992.
[3.] T. Erdogan, Circular symmetric, distributed feedback structures for surface emitting semiconductor lasers, PhD thesis, 1992.
[4.] M. Fallahi, F. Chatenoud, I. M. Templeton, M. Dion, C. M. Wu, A. Delage, and R. Barber, "Electrically pumped circular-grating surface-emitting DBR laser on InGaAs strained single-quantum-well structure," IEEE Photon Tech. Lett., vol. 9, pp. 1087-1089, 1992.
[5.] M. Toda, "Single-mode behavior of a circular grating for potential disk-like DFB lasers," IEEE J. Quantum. Electron., vol. 26, pp. 473-481, 1990.
[6.] U.S. Pat. No. 4,140,362 to P. K. Tien, "Forming focusing diffraction gratings for integrated optics," 1976.
[7.] U.S. Pat. No. 3,970,959 to S. Wang, "Two dimensional distributed feedback devices and lasers," 1976.
8.] H. Kogelnik and C. V. Shank, "Double-heterostructure GaAs distributed-feedback laser," Appl. Phys. Lett., vol. 25, pp. 200-201, 1974.
9.] W. L. Bragg, Proc. Cambridge Phil. Soc., vol. 17, p. 43, 1913.
10.] M. Born and E. Wolf, Principles of Optics, 3rd Ed. (Pergamon, N.Y. 1965).
11.] M. J. Adams, An Introduction to Optical Waveguides, (John Wiley and Sons, Chichester, 1981), Ch. 7.
U.S. Pat. No. 4,743,083 to R. M. Shimpe discloses designs of cylindrical diffraction grating couplers and distributed reflectors for guided wave devices. These designs use periodic concentric gratings to control cylindrical waves, in much the same way as one-dimensional gratings to plane waves. A major part of our inventions--distributed reflectors and couplers--improves on Shimpe's designs in using non-periodic structures spaced according to specific rules.
T. Erdogan et al. achieved the first optically-pumped surface-emitting semiconductor laser using a periodical grating design. Ref. [2] and [3] show their best results in detail. Ref. [4] demonstrated one of the first electrically-pumped surface-emitting laser using periodic gratings. These references describe in detail how these lasers are constructed. They also discuss the potential use of these devices for high-power laser sources.
Ref. [5] proposes a design that places the grating steps at their roots of Bessel/functions, the nodes of the standing waves. However, it does not take into account of the waves' phase changes at the ridge-groove-step interlaces.
U.S. Pat. No. 4,140,362 to P. K. Tien discloses techniques for producing periodically spaced curved gratings to couple and focus optical waves from waveguides.
U.S. Pat. No. 3,970,959 to S. Wang et al. describes the use of two series of straight line (one-dimensional) gratings to reflect guided light waves and to couple light into and out of waveguides.
Kogelnik et al. describe how one-dimensional DFB lasers can achieve narrow linewidth. The paper by W. L. Bragg explains how periodic structures reflect X rays.
The book by Born et al. teaches how multi-stack DBRs and anti-reflective layers can be constructed and used. It and the book by Adams are excellent textbooks for analyzing optical wave devices.
However, all prior designs of cylindrical wave controls fail to perform well because their grating spacings do not match well with the phases of the cylindrical waves. For instance, surface-emitting semiconductor lasers with periodic grating cylindrical DBR and DFB structures, demonstrated in Refs. [2] and [3], have yielded spectra tar from pure; and the optical-power distributions in the structures are far from what are desired for practical uses.
For these structures, the grating-step spacing design is critical. The spacings affect the phases of the transmitted, reflected and diffracted waves at medium interfaces, and control the amounts of these waves. The behavior of cylindrical waves are more complex than that of plane waves. Most of the prior designs simply adopt the argument that is used for plane: waves and assume that phases of cylindrical waves change 90 degrees at every quarter wavelength; but in fact cylindrical waves are not periodic in the radial direction.
Only one prior design (Ref. [5]) improves on the periodic design and argues that phases of cylindrical waves change at the roots of Bessel functions; however, cylindrical-wave phase changes at media inter/aces are not counted in this argument.
Our invention takes into account both the wave-propagation phase change and the phase change at media interfaces. With this accurate method for controlling the wave phases, we design cylindrical-wave controlling devices and elements for wave-controlling, generating and guiding devices. Our invention improves on the designs in U.S. Pat. No. 4,743,083 and other prior designs. We outline specific design rules for determinating the grating-step spacings. In addition, our invention extends the underlying principles to the design of other wave-control devices and to the design of semiconductor lasers of narrow linewidth and high power. The principles apply to both guided cylindrical waves and unguided (conventional) waves.